Abstract: The invention provides an internal pollution preventing apparatus for a handpiece wherein a valve itself is small and compact and a housing is small in size and which is small in number of parts and superior in assembling facility, airtightness and economy.The apparatus has an air turbine drive air circuit A for supplying, when the handpiece operates, air of a comparatively high pressure via a valve 1, an exhaust air circuit D for exhausting air after having driven an air turbine of the handpiece, a chip air circuit B for supplying air for the atomization of cooling water via valves V1, V2, V3, V4 and V5, and a low pressure air feeding circuit E for feeding air turbine stopping and circuit internal pressure keeping air of a low pressure to the drive air circuit and the chip air circuit via further valves.